Hitherto, retransmission methods such as hybrid automatic repeat request (HARQ) have been known in which a base station performs retransmission processing on a packet to a mobile terminal.
For example, when a packet received by a mobile terminal contains an errors that are difficult for the mobile terminal to correct the errors, the base station implementing HARQ may retransmit the packet or transmit a redundancy bit that has not been transmitted with puncturing. The term “puncturing” refers to transmission of thinned-out information.
Then, for example, the mobile terminal implementing HARQ may hold the packet containing errors. When the mobile terminal receives the packet or redundancy bit transmitted by the retransmission processing from the base station, the mobile terminal may synthesize the received packet or redundancy bit and the held packet and correct the errors. For example, the synthesis of the held packet and the received redundancy bit by the mobile terminal can improve the error correction performance for the held packet to correct the errors.
According to the retransmission method, a base station uses the transmission condition used for the first transmission to perform the retransmission processing. For example, a base station may use the equal transmission power to the transmission power used for the first transmission to perform the retransmission processing. Generally, as the transmission power increases, the amount of error decreases in the packet transmitted with the transmission power and received by a mobile terminal. As the transmission power decreases, the amount of error increases in the packet transmitted with the transmission power and received by a mobile terminal.
It has been known that fading and rapid changes in radio-wave propagation state occur while a mobile terminal is moving at a high velocity. Methods have also been known in which a radio parameter is changed in accordance with quality of service (QoS). Those methods are disclosed in Japanese Laid-open Patent Publication No. 2005-86304 and Japanese Laid-open Patent Publication No. 2008-295070.
However, those retransmission methods in the past may repeat ineffective retransmission processing. More specifically, according to a retransmission method in the past, while a mobile terminal is moving at a high velocity and when the radio-wave propagation state rapidly deteriorates, even repeated retransmission processing on the target packet may not allow the mobile terminal to fully correct errors in the packet and does not result in the error correction. In other words, the retransmission processing is performed ineffectively.
For example, the retransmission method may use the transmission power used for the first transmission to repeat the retransmission processing even when the radio-wave propagation state is deteriorated. However, the retransmitted packet received by the mobile terminal and transmitted by the retransmission processing may contain many errors that may be difficult for the mobile terminal to fully correct like the packet received by the mobile terminal in the first transmission.
For example, when the radio-wave propagation state of a mobile terminal rapidly deteriorates, errors may occur. In such a case however, the mobile terminal may not fully correct the errors even with the redundancy bit that has not been transmitted with puncturing, retransmitted by the base station and received by the mobile terminal.